1. Field of the invention
The invention concerns a dimmer for a load such as an incandescent lamp connected to an alternating current mains power supply and adapted to be inserted into a conductor supplying power to the load via input and output terminals and comprising a triac with two main terminals of which a first is connected to the input terminal and the second is connected to the output terminal, a control circuit sending to the triac a triggering signal synchronized to the voltage between the terminals with a variable time-delay, a filter inductor with one end connected to the second main terminal of the triac and a second end connected to the output terminal and a filter capacitor connected between the input and output terminals.
Dimmers comprising a triggered conduction switching device and a control circuit adapted to trigger conduction with a variable time-delay from the zero crossing of the voltage across the switching device have been known in themselves for a long time. The development of semiconductor devices, thyristors and triacs mass produced at low cost has made it possible to extend the use of such dimmers to domestic applications such as controlling the brightness of light fittings.
The principle of controlling the power supplied to a load by the variable time-delay triggering of the switching device in which the impedance of this device changes in a time that is very short compared to the period of the AC line voltage from a value that is very high compared with the load impedance to a value that is negligible compared to this impedance causes significant discontinuity in the voltage across the load. Unless special precautions are taken, especially if the load absorbs essentially active power, each triggering causes a transient current demand on the mains power supply. These transients constitute undesirable interference.
Dimmers are therefore provided with lowpass type filters conventionally comprising a series inductor and a capacitor shunting the load. The values of these components are determined in relation to the impedance of the load so that the impedance of the inductor is low compared to the load impedance at the AC line voltage frequency and high compared to the load impedance at the harmonic frequencies caused by triggering the switch device while the impedance of the capacitor is respectively high and low at these same frequencies.
Although the losses at the frequencies in question, which range from 50 Hz to a few tens of kHz, may be negligible in the case of capacitors, this is not so in the case of inductors whose internal resistance cannot be eliminated, even by winding them on toroidal ferromagnetic cores. These losses represent a significant part of the power dissipated by the dimmer.
As the size of a component must increase with the power that it dissipates to prevent excessive temperature rises and since the price of an inductor increases in proportion to its size, in dimmers for domestic applications at least the filter inductor becomes the most costly, bulky and power dissipating component.
The invention is therefore directed to reducing losses in the filter inductor to reduce the cost and size for the same load power or to increase the power rating for the same cost and size.